Conventionally the internal bearing of a brushless motor includes:    a ball bearing having a shaft supported by a ball bearing    a plain bearing having a shaft supported via the sleeve of a sintered metal    a dynamic pressure oil bearing known as a fluid bearing Brushless motors having dynamic pressure oil bearings are used in latest drives which rotate disk recording mediums at high speed.
Japanese Patent Laid-Open No. 2003-239949 discloses a spindle motor for driving a hard disk. In the spindle motor, a porous material (oilless bearing) for containing lubricating oil is used for a sleeve and a groove for a dynamic pressure is formed on a shaft.
Further, Japanese Patent Laid-Open No. 2001-124057 discloses a motor constituted of a dynamic pressure fluid bearing in which an oilless bearing is used for a sleeve and a herringbone groove is formed on a shaft on the output side of the sleeve.
Japanese Utility Model Laid-Open No. 59-164822 discloses a so-called hybrid bearing which is constituted of an oilless bearing made of a sintered porous material, so that a merit of a hydrodynamic bearing and a merit of an oilless bearing are obtained. In this configuration, a groove for generating a dynamic pressure is formed on the sintered porous material constituting the bearing, and crushing porous surface is performed on the bottom and sides of the groove for generating a dynamic pressure in order to support a dynamic pressure. Crushing porous surface is not performed on a part other than the groove for generating a dynamic pressure.
In order to solve the problem of Japanese Utility Model Laid-Open No. 59-164822, Japanese Patent Laid-Open No. 10-196646 discloses a hydrodynamic bearing device which preferably obtains both of an oilless bearing function and a dynamic pressure bearing function over the long term. In the configuration of Japanese Utility Model Laid-Open No. 59-164822, crushing porous surface is not performed on a part other than the groove for generating a dynamic pressure, so that a dynamic pressure of several to ten atmospheres presses lubricating oil in the bearing to the outside through sintered holes and the dynamic pressure of the bearing tends to gradually decrease. When bubbles occurs in the bearing or bubbles expand due to a temperature rise or a pressure reduction, bubbles compressed by a dynamic pressure easily enter the sintered holes and increase the leakage of lubricating oil from the oilless bearing. For this reason, in Japanese Patent Laid-Open No. 10-196646, the part of bearing for generating a dynamic pressure is larger in capillary force than the sintered hole of an oil impregnated sintered bearing.
Japanese Patent No. 3441695 and others disclose techniques of forming a dense part and a rough part with different aperture ratios inside an oilless bearing (the above-described sleeve)
Motors with so-called hybrid bearing structures are disclosed in Japanese Patent Laid-Open No. 2003-239949, Japanese Patent Laid-Open No. 2001-124057, Japanese Utility Model Laid-Open No. 59-164822, and Japanese Patent Laid-Open No. 10-196646. The motor uses an internal bearing formed by combining a shaft having a groove for generating a dynamic pressure and an oilless bearing. Such motors are suitable for high-speed and constant-speed operations but are not suitable for uses in changing a rotation speed over a wide range due to frequent start and stop of high-speed and constant-speed operations. Thus, it is quite difficult to design dynamic pressure fluid bearings.
As disclosed in Japanese Utility Model Laid-Open No. 59-164822 or Japanese Patent Laid-Open No. 10-196646, when the groove for a dynamic pressure is formed on the inner surface of the sleeve made of a porous material to increase the dynamic pressure of a lubricating fluid of a radial bearing, the dimensions and shape of the sleeve are limited by the worked surface of the groove for a dynamic pressure. Thus, such a configuration is not preferable. Particularly in the hybrid bearing of Japanese Patent Laid-Open No. 10-196646, working is necessary to make the part of bearing for generating a dynamic pressure larger in capillary force than the sintered hole of the oil impregnated sintered bearing, thereby increasing restrictions. Since the groove for a dynamic pressure on the inner surface of the sleeve is made of a porous material, it is necessary to consider durability and workability for an inspection and a test of the groove for a dynamic pressure because the groove is disposed on the inner surface of the sleeve.
As disclosed in Japanese Patent Laid-Open No. 2003-239949 and Japanese Patent Laid-Open No. 2001-124057, when the groove for a dynamic pressure is formed on the outer surface of the shaft, it is possible to solve the problem of Japanese Patent Laid-Open No. 2003-239949 or Japanese Patent Laid-Open No. 2001-124057, in which the groove for a dynamic pressure is disposed on the sleeve of a porous material. However, the brushless motor which requires changing a rotation speed over a wide range faces another problem. To be specific, as shown in FIG. 6A, in the dynamic pressure fluid bearing, a gap 3 is formed between a sleeve 1 and a shaft 2, herringbone grooves 4a and 4b are formed on the shaft 2, the gap 3 is filled with lubricating oil 5, the lubricating oil 5 flowing through the herringbone grooves 4a and 4b gathers between the herringbone grooves 4a and 4b and the inner surface of the sleeve 1 due to the high-speed rotation of the shaft 2 and constitutes fluid bearing mechanisms 6a and 6b, the shaft 2 is supported and rotated while being kept from contact with the inner surface of the sleeve 1 to ensure good performance of the bearing during high speed rotation. However, at low rpm during start-and-stop operations, as shown in FIG. 6B, the shaft 2 oscillates with respect to the sleeve 1. The oscillation brings the shaft 2 into contact with ends 7a and 7b of the sleeve 1 and degrades low-speed performance. In addition, the sleeve may be worn and cause seizing.
Particularly in the case of Japanese Patent Laid-Open No. 2001-124057, since the herringbone groove is formed on the output end of the shaft, it is necessary to fill sintered holes on the inner surface of the sleeve facing the herringbone groove to prevent lubricating oil from being pressed back into the sleeve, so that the function of the oilless bearing is lost. Hence, after start-and-stop operations are repeated, the sleeve facing the herringbone groove is like to wear and cannot stand many hours of operation.